In a conventional optical disk apparatus, when reproducing data, a light beam of a relatively weak constant light amount is radiated onto an optical disk as an information medium, and reflected light which has been modulated by the optical disk to become more or less intense is detected, thus achieving reproduction. When recording data, the light amount of a light beam is modulated to become more or less intense in accordance with a signal to be recorded, thus writing data to a film of recording material on the optical disk. In a read-only optical disk, information in the form of pits is previously recorded in a spiral form. A write-once or rewritable optical disk is produced by forming a film of material that enables optical recording and reproduction on the surface of a base having a spiral form of tracks with bump-and-dent structure, with a technique such as vapor deposition.
A laser light source is generally employed in forming recording marks on an optical disk. In order to form good recording marks, it is necessary for a laser light source to emit light with a predetermined waveform. In order to form recording spaces, a laser light source is driven to continuously emit light at a constant power for a predetermined time. In order to obtain a stable recording performance, the power of such a laser light source must be accurately controlled. However, the power characteristics of a laser light source are greatly influenced by ambient temperature and the like. Therefore, even if a power is set prior to recording and a constant driving current is supplied, a constant power will not be maintained, and power fluctuations will occur due to an increase in the temperature of the main body of the laser light source or the peripheral devices.
An operation of a conventional optical disk apparatus will be described with reference to FIG. 12, FIG. 13, FIG. 14, and FIG. 15.
FIG. 12 shows an example of an emission waveform from a laser light source at recording. FIG. 13(a) shows a layout of test areas in a DVD-RAM, and FIG. 13(b) shows a layout of test areas in a BD (Blu-ray Disc). Among emission patterns of a laser light source, FIG. 14(a) shows an exemplary test emission pattern not containing multipulses, whereas FIG. 14(b) shows an exemplary test emission pattern containing multipulses.
FIG. 15(a) shows an exemplary test emission pattern in the case of 4×. Herein, Mx means that the speed with which the optical disk is accessed is an M times speed. FIG. 15(b) shows an exemplary test emission pattern in the case of 5×; FIG. 15(c) shows an exemplary test emission pattern in the case of 6×; FIG. 15(d) shows an exemplary test emission pattern in the case of 7×; and FIG. 15(e) shows an exemplary test emission pattern in the case of 8×.
A laser emission waveform when recording is composed of a combination of a plurality of powers (see, for example, Patent Literature 1). FIG. 12 shows a laser emission waveform when recording. In a space portion, light is emitted at a space power 120s. In a mark portion, light is emitted with a combination of a peak power 120p, a mark power 120m, and a cooling power 120c. Laser power control at recording is applied to all of the aforementioned powers.
An effective method of avoiding power fluctuations is performing a power calibration per every constant period. For example, in an optical disk format having a sector structure as shown in FIG. 13(a), one test area for correcting laser power is provided per sector. In this case, as the focal point of a light beam passes through a test area, laser power control based on test emission is performed in that test area, thus avoiding power fluctuations.
In an optical disk format that lacks test areas, it is necessary to perform laser power control in a data area, in which data recording is to occur. In that case, the peak power and the cooling power, whose emission times are short, are difficult to be detected especially in the case of a high linear velocity, and thus control errors tend to increase.
Hereinafter, any linear velocity will be expressed as an x-speed with respect to a standard linear velocity according to specifications.